Move overlay wiring out from the ready block

[tomivirkki]

All the code in this block is ignored if this._overlayElement.content doesn't happen to exist when ready() is invoked: https://github.com/vaadin/vaadin-dropdown-menu/blob/master/src/vaadin-dropdown-menu.html#L340

And since ready() is invoked only once, the component can remain broken for the rest of its lifespan.

Add a MutationObserver to the component instead and do the wiring only once a template or a renderer is received.

[web-padawan]

Note: also breaks the renderer function when using in lit-element
https://glitch.com/edit/#!/zesty-bumper?path=app.js:35:11